1. Field of Invention
The present invention relates to an optical system for a scanner. More particularly, the present invention relates to a concave mirror optical system for a scanner.
2. Description of Related Art
Rapid progress in multi-media technologies has lead to great advances in image-processing devices. In a few years time, the black-and-white palm-top scanners has developed into full-color high-resolution scanner system that can display fine detail with added realism.
In general, scanning systems can be roughly divided into the reflective type and the transparent type. In the reflective scanning system, a document (made of non-transparent material) is placed on a transparent glass panel. Light transmitting through the transparent glass panel impinges upon the document. The light is reflected by the document into an optical system before delivering to an optical sensor such as a charge couple device (CCD) or a contact image sensor (CIS) to form a scan image. In the transparent scanning system, a document (made of transparent material such as a film negative or projector slide) is also placed on a transparent glass panel. However, light form a light source penetrates the document after passing through the transparent glass panel to form a scan image at the optical sensor.
FIG. 1 is a schematic cross-sectional view showing a conventional scanning system. To scan a non-transparent document 40 on a transparent glass panel 20, the reflective scanning system is activated. A light source 38 on a sense carrier 30 aims a beam of light at the transparent glass panel 20. The beam of light is reflected back by the document 40 and is channeled through a slit 34 into the sense carrier 30. An optical system 36 (details not shown) inside the sense carrier 30 picks up the reflected light and transmits the light to an optical sensor 32 (a CCD or a CIS). Inside the optical sensor 32, data encoded in the light beam is transformed into image data. Hence, an image line is scanned. As the sense carrier 30 moves forward along the transparent glass panel 20, different swath of image comes into view. Each swath is captured by sequentially until image data of the entire document is captured. When the scanning system operates in the reflective mode, a light source carrier 10 above the glass panel 20 is stationary.
To scan a transparent document 40, the transparent scanning system is activated. A light source 12 inside the light source carrier 10 produces a beam of light through a slit 14 and aims at the document 40 on the transparent panel 20. After passing through the document 40, the beam of light is channel through the slit 34 into the optical system 36 (detail not shown) of the sense carrier 30. The light passes from the optical system 36 into the optical sensor 32 where data encoded in the light beam is converted to a line of image data. Hence, an image line is scanned. As the light source carrier 10, and the sense carrier 30 moves forward along the transparent glass panel 20, different swath of image comes into view. Each swath is captured sequentially until image data of the entire document is captured. When the scanning system operates in the transparent mode, the light source 38 on the sense carrier 30 does not emit any light.
FIG. 2 is a sketch of the optical system inside a conventional scanner. As shown in FIG. 2, the purpose of an optical system 136 is to capture light image produced by the document 40 and relay the light image to the optical sensor 132. The optical system 136 includes at least a reflecting mirror 137 and a lens assembly 138. The reflecting mirror 137 is used to change light travel direction while the lens assembly 138 is used for focusing the light onto the optical sensor 132.
In a conventional scanner, a reflecting mirror 137 and a lens assembly 138 are generally used inside the optical system 136. Hence, the optical system is structurally complicated and difficult to assembly, thereby increasing the production cost. Moreover, the lens assembly 138 may produce chromatic dispersion that may affect the image quality.